164 INTRODUCTION TO CYTOLOGY 



tetraploid number, in all of their cells, (e) In the threadworm, Ascaris 

 megalocephala, fertilization of an egg of the variety bivalens (two chromo- 

 somes) by a spermatozoon of the variety univalens (one chromosome) 

 resulted in a larva with three chromosomes in all its cells, the chromosome 

 contributed by the male parent being distinguishable from the other two 

 (Boveri 1888a; Herla 1893; Zoja 1895). 



Results such as the above led Boveri to the conclusion that the number 

 of chromosomes arising from the reticulum in prophase is directly and 

 exclusively dependent upon the number that went to make it up in the 

 preceding telophase. If a nucleus is reconstructed in the telophase by an 

 abnormal number of chromosomes as the result of a disturbance of the 



10 9 



FIG. 57. The chromosome complement of Hesperotettix viridis. 



A, the 12 bivalent chromosomes of the spermatocyte, including the accessory chromo- 

 some (No. 4.) B, complement from another individual, showing two "multiple chromo- 

 somes." Nos. 11 and 12 have united temporarily, as have also Nos. 4 and 9. X 1800. 

 (After McClung, 1917.) 



mitotic process, the altered number invariably appears in the succeeding 

 prophase: if extra chromosomes are present they are not eliminated in 

 any way during the resting stages, and if chromosomes have been lost 

 during abnormal mitosis they are not replaced. These conclusions have 

 been strikingly confirmed by Sakamura's (1920) work on cells subjected 

 to the influence of chloral hydrate and other agencies causing aberrant 

 chromosome behavior. 



Variations in Number. Although the number of chromosomes in a 

 given species is on the whole remarkably constant, departures from nor- 

 mal numbers are occasionally observed. Strasburger (1905) believed 

 that the number, though determined by heredity, is not so rigidly fixed 

 that all variation in the vegetative cells is excluded ; only in the reproduc- 

 tive cells did he hold constancy in number to be necessary. Much light 



